Technical Field
The invention relates to GNSS/INS navigation systems and, in particular, to GNSS/INS navigation systems that utilize a yaw rate constraint during dead reckoning.
Background Information
GNSS/INS navigation systems, such as the systems described in U.S. Pat. Nos. 6,721,657 and 7,193,559, which are assigned to a common assignee and incorporated herein by reference, may be used in vehicles to aid in navigation and also in the control of the vehicles. The GNSS/INS navigation systems work well to provide accurate up-to-date INS-based position, velocity and attitude information to a vehicle navigation and control system that, in turn, provides navigation information to a user directly or via a map. Alternatively, or in addition, the vehicle navigation and control system may utilize the navigation information to control the vehicle.
A GNSS subsystem of the GNSS/INS navigation system receives GNSS satellite signals and, operating in a known manner, processes the GNSS satellite signals to produce GNSS measurements and observables and determine GNSS positions, velocities, times and associated covariance corresponding to GNSS measurement times. An INS subsystem includes an inertial measurement unit (IMU) that reads data from orthogonally positioned sensors, i.e., accelerometers and gyroscopes, and produces IMU measurements. As is known, the accelerometers and gyroscopes have associated biases that, if not corrected for, introduce drift errors into the IMU measurements. The INS subsystem, operating in a known manner, uses the GNSS position, covariance and, as appropriate, GNSS measurements and observables in an INS filter to aid in the correction of the drift errors in the INS measurements. The inertial subsystem then utilizes inertial measurements to provide INS-based position, velocity and attitude in between the GNSS measurement times, that is, during measurement intervals.
The GNSS/INS navigation system operates in a known manner to provide uninterrupted navigation information, even in environments in which sufficient numbers of GNSS satellites are not continuously in view. When updated GNSS position, velocity, and time, and associated covariance and applicable GNSS measurements and observables are not available, the GNSS/INS navigation system utilizes the inertial measurements to continue to provide updated INS-based position, velocity and attitude during the GNSS outage. The operations of the GNSS/INS navigation system without GNSS updates are referred to as dead reckoning.
The gyroscopes in the IMU, particularly in a relatively low cost IMU, tend to have very large biases and can drift quickly when left un-aided by the GNSS information. Accordingly, without the GNSS updates, the accuracy of the INS-based position, velocity and attitude is adversely affected. In particular, uncorrected gyroscope measurements result in determinations of inaccurate vehicle heading which is one of the major sources of error in the calculations of INS-based position, velocity and attitude.
The GNSS/INS navigation systems work well when the GNSS outages are relatively short, such as, for example, when the systems momentarily loose lock on the GNSS satellite signals or fewer than a required number of GNSS satellites are in view because of obstructions in the area or traveling through tunnels and so forth, and aiding is again available to control the drift errors after a single or a small number of measurement intervals. When the outage of GNSS information lasts for much longer periods of time due, for example, to the GNSS antenna falling off of the vehicle or the GNSS antenna otherwise becoming disabled, or interference such as jamming in the area, the GNSS/INS navigation system must continue to operate with inertial sensors that have unchecked drift errors over many measurement intervals. The increasingly larger drift errors adversely affect the accuracy of the operations of the system and, in particular, result in increasing inaccuracies in the determination of vehicle heading. This in turn leads to greater errors in the calculation of the INS-based position, velocity and attitude.
One way to aid in the determination of the vehicle heading during dead reckoning is to equip the vehicle with specialized instrumentation, such as, for example, a magnetometer and/or a turning angle feedback instrument, to provide additional information for use in the calculation. However, the specialized instrumentation adds to the cost and complexity of the vehicle, and thus, such instrumentation is not typically included. Accordingly, the drift errors of the INS sensors and the associated inaccuracies in the calculation of position, velocity and attitude may result in the vehicle navigation and control systems determining that the vehicle has turned, when the vehicle is actually proceeding in the same direction. Alternatively, the control system may erroneously determine that the vehicle has not turned when, in fact, the vehicle has turned. Either inaccuracy adversely affects the operations of vehicle navigation and/or vehicle control by the navigation/control system.